[Wien] query for elastic constants
fatima DFT
fatimadft5 at gmail.com
Wed Sep 29 03:17:17 CEST 2021
Dear Peter Sir,
Please have a look at my recent query.
Regards
Fatima
On Mon, Sep 27, 2021, 6:19 PM fatima DFT <fatimadft5 at gmail.com> wrote:
> Dear Sir,
> Thank you very much for your reply.
> Sorry for the late response as I was doing phonon calculation with LDA.
>
> I found that with LDA, the system shows thermodynamic stability, and
> that's why I was getting all frequency positive at compressive strain when
> I was using PBE.
>
> Can I proceed with PBE now with the lattice parameters I got with LDA or
> now I should use LDA only?
>
> I would be glad if I can get any reference on such a strategy where people
> use lattice parameters from experiments or from some functionals and then
> procced with other functional for further calculation.
>
> Thanks and regards
> Fatima
>
>
> On Sat, Sep 18, 2021 at 11:54 AM Peter Blaha <pblaha at theochem.tuwien.ac.at>
> wrote:
>
>> The most fundamental procedure is to use the theoretically optimized
>> volume for all properties.
>> If the DFT result is very bad, sometimes one also used experimental
>> lattice parameters.
>>
>> Phonons: There is nothing unusual about imaginary phonons in the
>> harmonic approximation for cubic perovskites. Your calculations are at
>> T=0 and most cubic perovskites are unstable at low temperature and show
>> one (or even more) phase transitions when T is lowered.
>>
>> In principle, one can do anharmonic phonons, which allows to calculated
>> phonons at finite temperature and then obtain a free energy. This way
>> you can get lattice parameters at finite temp. as well as phase
>> transition temperatures.
>> see eg. PHYSICAL REVIEW B 103, 094108 (2021)
>>
>> Am 17.09.2021 um 13:52 schrieb fatima DFT:
>> > Dear Sir,
>> > I understand your point. I am using PBE functional.
>> >
>> > I am still in doubt, which lattice parameter I should use for the rest
>> > of the properties.
>> > Thanks
>> >
>> >
>> > On Fri, Sep 17, 2021 at 5:11 PM Laurence Marks <
>> laurence.marks at gmail.com
>> > <mailto:laurence.marks at gmail.com>> wrote:
>> >
>> > That means that the cubic phase is not stable at T=0 for the
>> > functional you have chosen. This may be correct or incorrect, it is
>> > not uncommon (e.g. SrTiO3 with PBE).
>> >
>> > _____
>> > Professor Laurence Marks
>> > "Research is to see what everybody else has seen, and to think what
>> > nobody else has thought", Albert Szent-Györgyi
>> > www.numis.northwestern.edu <http://www.numis.northwestern.edu>
>> >
>> > On Fri, Sep 17, 2021, 06:37 fatima DFT <fatimadft5 at gmail.com
>> > <mailto:fatimadft5 at gmail.com>> wrote:
>> >
>> > Thanks Sir
>> > No Sir!!!
>> > I mean, the original system is lower in energy but the phonon
>> > band structure has some soft modes.
>> > When I apply a compressive strain, the soft mode disappears.
>> > I am sorry if you found my previous query a bit confusing.
>> >
>> > Thanks
>> > Fatima
>> >
>> >
>> >
>> > On Fri, Sep 17, 2021 at 5:01 PM Laurence Marks
>> > <laurence.marks at gmail.com <mailto:laurence.marks at gmail.com>>
>> wrote:
>> >
>> > If I understand you correctly, your system is lower in
>> > energy for a slightly smaller lattice parameter. This is not
>> > unusual, and depends upon what functional you use. It is
>> > standard to calculate using the cell parameters that
>> > minimize the dft energy.
>> >
>> > Is this right? What is the relevance of "right"? Doing
>> > phonons for a non dft-equilibrium cell is obvious GIGO.
>> > Beyond that to band etc there is no "right".
>> >
>> > _____
>> > Professor Laurence Marks
>> > "Research is to see what everybody else has seen, and to
>> > think what nobody else has thought", Albert Szent-Györgyi
>> > www.numis.northwestern.edu <
>> http://www.numis.northwestern.edu>
>> >
>> > On Fri, Sep 17, 2021, 06:22 fatima DFT <
>> fatimadft5 at gmail.com
>> > <mailto:fatimadft5 at gmail.com>> wrote:
>> >
>> > I want to extend my question for band, doss, optical
>> > properties as well.
>> >
>> > One important thing is that the total energy is more
>> > negative for the original system by 0.65 eV when I apply
>> > 2.5% compressive strain and 2.8 eV when I apply 5%
>> > compressive strain.
>> >
>> > Thanks
>> >
>> >
>> > On Fri, Sep 17, 2021 at 3:33 PM fatima DFT
>> > <fatimadft5 at gmail.com <mailto:fatimadft5 at gmail.com>>
>> wrote:
>> >
>> > Dear All,
>> >
>> > I am dealing with a cubic 3D perovskite system.
>> > With its original lattice parameters, the system
>> > shows negative frequencies.
>> > At some 1% lattice strain, the system becomes
>> > dynamically stable (no +ve frequency).
>> >
>> > My question is: Should I calculate elastic constant
>> > at the lattice parameters where the system is
>> > dynamically stable or should I consider the case
>> > with its original lattice parameters (fully
>> > optimized geometry with no strain)?
>> >
>> >
>> > Thank you
>> > Fatima
>> >
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>> --
>> --------------------------------------------------------------------------
>> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
>> Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
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